The very first Greenpeace voyage in 1971 aimed to halt nuclear tests at Amchitka by sailing into the restricted area. Photograph: Robert Keziere/Greenpeace

Within a few days the tiny island of Amchitka in the Aleutians will be rocked by a five-megaton underground weapons test. The nuclear blast, codenamed Cannikin, and the largest ever undertaken by the US, takes place in the face of official protests from Canada, Japan, and the State of Alaska, and against the wishes of a large proportion of the US scientific community.

After months of delay, approval for the test appears to have been given by President Nixon with unseemly haste and within a few hours of the defeat of the US in the UN over the admission of China. The official reason for going ahead, "because it is in the national interest" of the US, will probably be condemned as wholly cynical by the organisations and individuals who have, for more than a year, fought for its abandonment.

The most telling arguments against the test, which is taking place on Russia's back doorstep, are not environmental. Cannikin was designed some years ago as part of the development programme for the "heavy" ABM "Safeguard" system which, for reasons of cost and technical immensity, was abandoned almost two years ago. The modified light ABM system that took its place calls for smaller and different warheads. When President Nixon canvassed the opinion of seven Government agencies his own Office of Science and Technology pointed out that the test was of only marginal technical usefulness. Of the seven agencies canvassed only two, the US Atomic Energy Commission and the Department of Defence, expressed approval.

Significantly Dr Harold Agnew, the new director of the AEC's weapons development and test organisation - the Los Alamos Scientific Laboratory - has endorsed the view that the test is technically weak, while the Federation of American Scientists has said, categorically and officially, that Cannikin is "obsolete." Yet, against this kind of open public assessment, and in spite of injunctions sought by the US Committee for Nuclear Responsibility and various other collected environmental and anti-war groups, President Nixon has given the nod.

The environmental doubts, if less telling, are real enough. Some 19 US underground tests are known to have vented accidentally and, although the Amchitka site is structurally tougher than that in Nevada, the possibility of accident and of widespread radioactive contamination must exist. This is true of all underground tests on a large scale wherever they are carried out. It is equally true that there is a possibility, however small, that such tests will trigger a major earth movement and cause damage or tidal waves at points remote from the site. True, the US AEC has already tested Amchitka - which sits on a seismic ridge - with a smaller explosion, and has found that aftershocks are small and shortlived.

Yet the cosy reassurance of an earlier test is no justification for going ahead with a test whose purpose has long since been overtaken. It begins to seem as though the reasons for delay were not, as had been supposed, partially environmental, partially technical, and partially political, because of Russian visits to Canada and China. They were wholly political. As the concrete and gravel is tamped down in the huge 6,000ft.-deep excavation, it also looks as though the final reason for going ahead is political. Not, however, on the basis of parity for, although Russia had a big bang a few weeks ago, tests are neither designed nor used that way. But as a gesture of political pique because of economic pressures from Japan and because of defeat in the UN. Cannikin, you could say, is an expression of US national disgrace.